Top drives for earth boring drilling rigs are employed to rotate the pipe string. The top drive has a rotary motor and is pulled up and down a derrick by a set of blocks. While running casing or drilling with casing, a pipe gripping mechanism may be secured to the drive stem or quill extending downward from the top drive. The pipe gripping mechanism has gripping elements that are moved radially into gripping engagement with either the inner or outer diameter of the casing string.
While running a casing string, the downward movement of the top drive depends on the apparent weight of the casing string and the pipe gripping mechanism supported by the top drive. If the weight being imposed on the top drive is adequate to pull casing string down the well, personnel on the drilling rig will control the rate of descent of top drive through a draw works brake.
In highly deviated wells, the apparent weight of the casing string being supported by the top drive will likely decrease as the casing string lengthens because of the friction of the casing string in the deviated well. The weight imposed on the top drive due to the weight of the casing string could theoretically become zero, stopping descent of the casing string. For operational reasons, one would always want the top drive and the upper section of the pipe gripping mechanism to be under tension. Otherwise, one might accidentally apply the full weight of the top drive onto the pipe gripping mechanism, causing extensive damage.